Respond to font size changes

src/fontSize.js

@@ -0,0 +1,50 @@
+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import dis from './dispatcher';
+import SettingsStore from './settings/SettingsStore';
+
+export class FontWatcher {
+    constructor(min_size, max_size) {
+        this._min_size = min_size;
+        this._max_size = max_size;
+        this._dispatcherRef = null;
+    }
+
+    start() {
+        this._setRootFontSize(SettingsStore.getValue("font_size"));
+        this._dispatcherRef = dis.register(this._onAction);
+    }
+
+    stop() {
+        dis.unregister(this._dispatcherRef);
+    }
+
+    _onAction = (payload) => {
+        if (payload.action === 'update-font-size') {
+            this._setRootFontSize(payload.size);
+        }
+    };
+
+    _setRootFontSize = size => {
+        let fontSize = this._min_size < size?size:this._min_size;
+        fontSize = fontSize < this._max_size?fontSize:this._max_size;
+        if (fontSize != size) {
+            SettingsStore.setValue("font_size", null, fontSize)
+        }
+        document.querySelector(":root").style.fontSize = fontSize + "px";
+    }
+}